Glass fiber (also referred to as glass filament) used as a composite material is manufactured by continuously forming and spinning using a molding apparatus called bushing (also referred to as a platinum heating container) generally having a substantially rectangular appearance. The bushing apparatus, which is arranged at the bottom of the container having a pot shape and having a function of temporally reserving a molten glass, is made of a heat resistant metallic material such as platinum, and has a multiple nozzle portions (or orifice portions), and presents an appearance such as a container. According to the bushing apparatus, temperature is controlled such that the molten glass has an optimum temperature at the top of the bushing nozzle, that is, the uniformly molten glass has a temperature near the temperature corresponding to high-temperature viscosity of 103dPa·s, and thereby the molten glass continuously flows out of the bushing nozzles and rapidly cooled to form and to be spined as a glass fiber.
In the case where the glass fiber is formed in this manner, when a liquidus temperature Ty of the molten glass is equal to or core than a spinning temperature, which serves as a glass forming temperature Tx, crystals causing devitrification in the vicinity of the bushing nozzle are easily precipitated in the molten glass. As a result, the bushing nozzle gets clogged which causes thread breaking that is also referred to as “break.” Therefore, the liquidus temperature Ty of the molten glass needs to be lower than the spinning temperature (equal to the forming temperature Tx) (i.e., a temperature difference ΔTxy−Tx−Ty>0). To increase the difference (ΔTxy) between the liquidus temperature Ty of the molten glass and the spinning temperature Tx as much as possible, the spinning temperature Tx can be increased. However, it is not preferable since it causes an increase in manufacturing cost due to the increase in energy required for the melting, or causes a problem of reducing the life span of attached equipments such as a bushing apparatus. Therefore, it is preferable that the temperature difference ΔTxy is increased and the forming temperature Tx is reduced.
On the other hand, in the manufacturing of glass fiber, the reduction of the boron (B) content in glass composition has been attempted, taking the environmental pollution problem into consideration. In addition, since the raw material serving us a boron source is expensive, it becomes important to decrease the boron content in glass composition in order to achieve reduction in the manufacturing cost of glass fiber. From this viewpoint, all Patent Documents 1, 2 and 3 try to achieve the object by limiting the glass composition.
Moreover, for the applications in a functional member requiring fine structural control, a fine-count glass fiber product is strongly in demand. For example, a printed wiring substrate or the like needs to drill or laser machine a through hole (referred to as via hole, via, inner via hole, blind via hole, or the like) of 0.1 mm or less, which connects any conductor layers provided through an insulating substrate. It has been found that it is preferable to use the fine-count glass fiber as a glass fiber constituting a base material in order to apply such high-precision machining to the substrate.
To spin the fine-count glass fiber, a nozzle diameter of the bushing can be reduced. However, the thinner the nozzle diameter is, the more likely the problem of creep deformation or the like of the nozzle is to occur, thereby leading to a problem of short durable time of a base plate of the bushing. To avoid these problems, Patent Documents 4 and 5 and other documents provide inventions in which the shapes of bushing or nozzle are restricted. Moreover, when the glass fiber is formed by the bushing in the above manner, the nozzle clogging causes fiber breakage and reduces production yield. Hence, the prevention of these is important. Patent Document 6 provides an invention in which a weir is arranged to prevent any heterogeneous foreign matters from flowing into nozzles.
Patent Document 1: Japanese Patent Unexamined Publication No. 2000-247684
Patent Document 2: Japanese Patent Unexamined Publication No. 2005-29465
Patent Document 3: Japanese Translation No. 2003-500330 of PCT International Application
Patent Document 4: Japanese Patent Unexamined Publication No. 5-279072
Patent Document 5: Japanese Patent Unexamined Publication No. 7-215729
Patent Document 6: Japanese Patent Unexamined Publication No. 9-142871